When patients suffering from a chronic respiratory decease are caused to inhale an oxygen gas, it is unsuitable to supply the oxygen gas continuously, because some resistance against the expiration of the patient is generated due to the pressure of oxygen gas supplied during the expiration, and there is a loss of oxygen gas not inhaled by the patient. For this reason, there has been developed such a technique that an oxygen gas is permitted to be supplied only upon the inhalation by detecting the breath timing of the patient.
For example, Japanese Patent Application Laid-open No. 270170/87 discloses the technique in which a breath sensor containing a pyroelectric element is disposed in a nose cannula attached to a nose of the patient to control the supply of oxygen gas in response to electric signals from the breath sensor. In this technique, the change in temperature in the nose cannula caused by the breath of the patient is detected by the pyroelectric element to generate corresponding electric signals. When the level of the electric signals exceeds a predetermined trigger level for the inhalation, the electromagnetic valve is opened to supply oxygen gas to the nose cannula. Also, when the level of the electric signal exceeds another trigger level for the Expiration, the electromagnetic valve is closed to interrupt the supply of oxygen gas.
In addition, Japanese Patent Application Laid-open No. 143082/88 discloses the technique further developed from the afore-mentioned technique, in which at least one electromagnetic valve is provided in a nose cannula to control the supply of oxygen gas by opening and closing operations thereof in response to electric signals generated from a breath sensor. In this technique, the supply of oxygen gas can be carried out with an excellent responsibility to the breath and the concentration of oxygen gas supplied can be kept stable.
Further, there is also known such an arrangement that a diaphragm-type high-sensitive pressure sensor is used as a sensor for detecting the breath, and a nose cannula and an oxygen source are coupled to each other through a three-way valve. When the pressure sensor detects the inhalation transmitted through the nose cannula, electric signals are generated to control the three-way valve. This arrangement enables the oxygen gas to be supplied through the nose cannula for a predetermined period of time.
On the other hand, there has been proposed an air pressure change detector comprising a strip-like piezoelectric element supported at one end thereof on a substrate which is accommodated in container having a vent hole at a position opposed to the piezoelectric element (Japanese Patent Laid-open No. 208827/92). In this air pressure change detector, an outside air is introduced into an interior of the container through the vent hole. When the pressure change in the outside air occurs, the pressure change is propagated to air in the container to cause the air to vibrate. The vibration of the air in turn induces the vibration of the piezoelectric element so that electric signals are generated therefrom, thereby enabling the detection of the change in air pressure. Especially, since the piezoelectric element is supported in a cantilever-like manner, the piezoelectric element is responsive to an extremely weak and low-frequency aerial vibration. By this arrangement, the air pressure change detector can detect an extremely small change in air pressure.
In the afore-mentioned techniques disclosed in Japanese Patent Laid-open Nos. 208827/92 and 143082/88, the breath sensor constituted by the pyroelectric element is disposed in the nose cannula and exposed to the expiration to accurately detect the temperature change occurring in association with the breath. However, since the pyroelectric element shows a low resistance to humidity or moisture, there is such a risk that if the pyroelectric element is used for a long period of time, the required performance thereof is lost. In addition, if the nose cannulae, which are usually handled as disposable items, are provided thereon with the breath sensor and signal conductors for such an exclusive use, there arises a problem that they are too expensive to be handled as disposable items, or there is a risk that breakage or burnout of the signal conductors are caused in association with repeated and frequent use thereof.
In the case where the diaphragm-type high-sensitive pressure sensor is used, extremely small pressure change (for example, on the order of 0.04 Pa) can be detected. However, since the diaphragm-type high-sensitive pressure sensor has an extremely low resistance to burst pressure, there also arises a problem that the sensor is readily damaged by pressure change frequently occurring in daily use, e.g., when the nose cannula is held in the mouth.
Further, in the case where the air pressure change detector having such a construction that the piezoelectric element is supported at one end thereof on the substrate accommodated in the container, is used and the nose cannula is coupled to the vent hole formed on the container at the position opposed to the piezoelectric element, the pressure change can be detected in response to the breath without a risk that the piezoelectric element is adversely affected by humidity or moisture. However, the aerial vibrations associated with both the inhalation and the expiration act similarly upon the piezoelectric element, so that the piezoelectric element are vibrated in an approximately similar manner upon the inhalation and the expiration. Therefore, it becomes difficult to distinguish the inhalation and the expiration from each other based on electrical signals generated from the piezoelectric element. This causes a problem that the supply of oxygen gas cannot be controlled only by the electric signals generated from the piezoelectric element.
Accordingly, it is an object of the present invention to provide a breath-synchronization control unit for use in a gas feeder, which is capable of surely detecting the inhalation to supply a gas, and free from breakage or failure even upon misuse thereof frequently occurring in daily use.